Multidentate Adsorbates for Self-Assembled Monolayer Films

Abstract

The spontaneous adsorption of organic molecules on a variety of planar and nonplanar substrates, that is, self assembly, can generate films just one molecule thick. These nanoscale, self-assembled monolayer (SAM) films have been extensively used to engineer surfaces with well-defined properties. Their utility has been demonstrated in a wide range of applications, including wetting, adhesion, lubrication, patterning, and molecular recognition. Many SAM systems have been investigated, but alkanethiols adsorbed on gold are the most successful combination. This pairing offers a variety of advantages, including the ability to tune precisely the interfacial properties of a surface through the well-established organic synthetic methodologies that have been developed for preparing custom ω-terminated alkanethiols. Alkanethiolate monolayers are moderately stable at room temperature; however, these films degrade over time and readily desorb upon moderate heating. This shortcoming limits the use of SAMs in applications involving elevated temperatures or harsh environments. Accordingly, new adsorbates with multiple bonding moieties have been created to enhance the stability and versatility of SAMs. In this Account, we examine a variety of multidentate adsorbate structures that have been used to generate SAMs on planar substrates and on nanoparticles. Each of these chelating adsorbates (bidentates and tridentates) has been designed to generate well-defined organic monolayer films with multiple attachment points to the underlying substrate. This bonding arrangement allows the formation of SAMs with enhanced stability through the entropy-driven "chelate effect". The research examined here demonstrates that multidentate adsorbates provide robust films: they enable the use of SAMs under conditions that are incompatible with SAMs derived from normal alkanethiols. Another advantage offered by multidentate adsorbates is the capacity for new paradigms in thin-film composition. In particular, appropriately designed chelating adsorbates can be engineered to have two or more chemically distinct terminal groups that are covalently linked to the same underlying headgroup, without adding steric bulk that might prove detrimental to the resultant assembly. This strategy allows the generation of homogeneously mixed multicomponent surfaces, overcoming the problem of phase separation or "islanding" that is pervasive when two or more chemically distinct adsorbates are used to form mixed SAMs. Such homogeneously mixed films offer the opportunity to fine-tune the interfacial properties of a substrate and to create unique heterogeneous interfaces that are well defined by the chemical composition of the tailgroups exposed at the surface. The insight derived from these studies opens the door to new uses for SAMs, both in surface engineering applications (such as corrosion resistance and soft lithographic patterning) and in the stabilization and manipulation of nanoparticles.

Quantum dot surface engineering: Toward inert fluorophores with compact size and bright, stable emission

“…By increasing the number of ligating groups per molecule, the equilibrium constant of ligand adsorption to the QD surface can theoretically increase exponentially [210, 211]. While quantitative studies are sparse, binding strengths have been observed to be slightly higher for ligands containing two thiols to gold surfaces [212] and to QDs [98]. However in molecular ligands, the flexibility needed to maximize binding affinity [210] may be difficult to achieve and the binding groups typically take up a larger footprint area on the QD surface which can reduce ligand packing density [213].…”

“…Compared with molecular multidentate ligands, the backbones can be more flexible and the number of binding domains can be much greater to allow improved attachment to the surface. Moreover, the density of hydrophilic domains and binding domains can be chosen independently to tune packing parameters and to append a variety of chemistries together on the same molecular backbone, as it is challenging to achieve a chemically mixed surface using molecular thiolates that may phase separate on surfaces [212, 216]. …”

An electrochemical approach to fabricate a heterogeneous mixed monolayer on planar polycrystalline Au and its characterization with Lateral Force Microscopy

“…There have been many investigations on the characterization and manipulation of alkylthiol SAMs on noble metals and the properties of the interface have been included in a variety of reviews [3][4][5][6][7][8]. Recently, the interest in Au-thiol chemistry has been renewed due its relation to nanotechnology as it was discovered that alkylthiols are efficient stabilizing ligands for gold nanoparticles (AuNPs) [9].…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…Alternatively, other types of adsorbates having multiple headgroup-metal interaction points have also been developed and studied [37][38][39]. Bidentate adsorbates have been introduced into surface chemistry due to their ability to enhance the stability of thin films formed on solid substrates [40].…”